The supply voltage of a voltage source is connected to the DC motor via the actuating circuit in accordance with prescribed parameters in order to regulate the power and/or speed of the DC motor. To this end, the actuating circuit uses a control arrangement, preferably what is known as pulse width modulation, which involves voltage pulses of variable width being supplied to the DC motor. In accordance with the pulse width of the voltage pulses, a speed is established on the DC motor.
An actuating circuit uses electronic switching elements such as transistors, thyristors, MOSFETs or similar components in order to be able to take prescribed parameters as a basis for setting the pulse width over the entire operating range of the DC motor. If an electronic component fails on account of a defect, two states can essentially arise.
If the defective component breaks the circuit permanently, the DC motor will no longer be able to be operated; if the defective component is conductive, then the DC motor can rotate even without the PWM actuation, the applied storage battery voltage acting on the DC motor in unregulated fashion. The electronic disturbance in the actuating circuit cannot readily be discerned by the user.